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Luminescence dating of sediments using individual mineral grains


Abstract and Figures

Luminescence is widely used to produce absolute ages for the time of deposition of a variety of types of sediments. The method relies upon the as-sumption that all grains are exposed to sufficient daylight prior to deposition for their luminescence signal to be reduced to a negligible level. Recent research has focused on the analysis of the luminescence signal from single mineral grains to produce an age. At this scale it is possible to identify different populations of mineral grains within a sample – some of which were bleached at deposition and some which were not. The methods involved in such analyses are discussed, and examples are given of depositional environments where this type of analysis is essential.
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GEOLOGOS 5 (2000)
PL ISSN 1426-8981 • ISBN 83-86682-45-0
Luminescence dating of sediments
using individual mineral grains
1 Institute of Geography and Earth Sciences, University of Wales,
Aberystwyth, UK
2 Nordic Laboratory for Luminescence Dating, Department of Earth
Sciences, University of Aarhus, Risø National Laboratory, DK-4000
Roskilde, Denmark
Abstract: Luminescence is widely used to produce absolute ages for the time of
deposition of a variety of types of sediments. The method relies upon the as-
sumption that all grains are exposed to sufficient daylight prior to deposition for
their luminescence signal to be reduced to a negligible level. Recent research has
focused on the analysis of the luminescence signal from single mineral grains to
produce an age. At this scale it is possible to identify different populations of
mineral grains within a sample – some of which were bleached at deposition and
some which were not. The methods involved in such analyses are discussed, and
examples are given of depositional environments where this type of analysis is
Key words: luminescence dating, laboratory technique, measurement protocols,
single grains, OSL.
Luminescence dating is now an important tool for obtaining absolute
age estimates for sedimentary deposits. Recent reviews (e.g. Duller 1996; Aitken
1998) have highlighted the increasing use of optically stimulated luminescence
(OSL) measurements instead of thermoluminescence (TL). OSL is much better
suited to dating geological materials since the same process of optical resetting
of the luminescence signal is used in the laboratory and in nature, and less
exposure to daylight is required to reset the OSL signal than TL. However,
another major advance in the last ten years has been the adoption of new meas-
urement procedures that have permitted analysis of samples using smaller amo-
unts of material.
Luminescence dating is based on the measurement of two quantities, the
radiation dose received by a sample since burial (the palaeodose, P) and the rate
at which it has absorbed energy from the natural environment (the dose rate). Of
these, it is the palaeodose that is derived from luminescence measurements.
Dividing one quantity by the other gives the period of time since deposition of
the sediment:
Age(years) = Palaeodose(Gy)
DoseRate(Gy/year) .
An essential assumption of the method is that the luminescence signal from
a grain can be reset, or zeroed, by exposure to daylight. This is a process com-
monly termed ‘bleaching’ and occurs in many processes of erosion, transporta-
tion or deposition, particularly those that occur sub-aerially. Prior to 1991, almost
all palaeodose measurements were made using many tens of thousands of grains
of a sample, spread between many tens of sub-samples, or aliquots. An implicit
assumption within all of these methods, known as multiple aliquot methods, was
that the luminescence characteristics of each aliquot were identical. This required
either a completely homogeneous sample, or the use of aliquots of a sufficient
size to average out any variations. Where there were significant differences
between the grains, possibly due to differing extents of bleaching at deposition,
this was seen as scatter in the data and resulted in uncertainty in the final age
estimate produced (Huntley & Berger 1985). More seriously, the presence of
a few very bright grains which had not been bleached at deposition could lead to
a significant overestimate of the age. For instance, Duller et al. (1995) showed
that a glacio-fluvial deposit from Scotland yielded an OSL age that was at least
five times older than independent age estimates. Within the last 10 years methods
have been developed that can measure the palaeodose from single aliquots and,
more recently, single sand-sized mineral grains. This has important implications
for the potential of luminescence dating and its reliability. This paper outlines the
procedures involved in such measurements, and their advantages.
Single aliquot methods
A number of luminescence workers have suggested that it would be
feasible to make all the measurements necessary to calculate a palaeodose on
a single aliquot. Duller (1991, 1994a) was the first to demonstrate how this could
be undertaken in practice, and these methods have been used widely. This was
an important development for a number of reasons. Firstly, for dating geological
sediments it meant that if a sample contains a mixture of grains, some of which
had been bleached at deposition and some of which had not, this could be
identified. In such a situation, different aliquots would contain different propor-
tions of well bleached and unbleached grains and hence give different palaeodo-
Fig. 1. Palaeodose plotted against the intensity of the natural signal for a set
of aliquots from (a) a well bleached sample (Bythe Lower Gravel – BLG) and
(b) a poorly bleached sample (Bythe Windermere Interstadial – BWI). In each
case the data were obtained using an additive dose single aliquot procedure
applied to potassium rich feldspars separated from late Quaternary fluvioglacial
sands from Scotland. Further details of the two sites are given in Duller (1994b).
Luminescence dating of sediments using individual mineral grains
ses (e.g. Li 1994). Only those grains whose luminescence signal was fully reset
at deposition would give an accurate palaeodose, and hence age; if a sample
contained some fraction of unbleached grains the palaeodose would be overesti-
mated, and so would the age. A number of different approaches have been
developed to analyse such data, both to identify those samples where this is
a problem, and in an attempt to provide an upper limit on the age estimate (e.g. Li
1994). The most important method has been to produce a scatter plot of the
values of the palaeodose and the OSL signal intensity from a number of aliquots
of the same sample. If a sample contains only well bleached grains then the
palaeodose derived from each aliquot will be similar, and there should be little
variation in signal intensity (Fig. 1a). In contrast, where a mixture of bleached
and unbleached grains are present, a wider range of palaeodoses will be obser-
ved, and those aliquots with the larger palaeodose will also tend to have a brigh-
ter luminescence signal (Fig. 1b). A linear regression line is placed through the
data, and if the slope of the line is significantly greater than zero then it is
deduced that the sample is incompletely bleached. This approach has been ap-
plied to fluvial, colluvial (Wintle et al. 1993) and glacial (Duller 1994b) sedi-
ments, but is equally applicable in other depositional environments where the
degree of optical bleaching at deposition is uncertain (e.g. coastal storm and
tsunami deposits, mass movements, soil forming processes). This form of analy-
sis relies upon an implicit assumption that within a sample all aliquots have
a similar sensitivity to radiation, and produce a fixed OSL signal per unit dose.
This assumption is reasonably valid for potassium-rich feldspars when analysing
many grains on an aliquot, and all the examples listed above conform to this
condition. However, for other materials such as quartz, or when the number of
grains in a sub-sample is small, this assumption is not valid and different me-
thods of analysis are required.
Results from small numbers of grains
Initial single aliquot work concentrated solely on the use of potas-
sium-rich feldspars as the dosimeter. However, Murray et al. (1995, 1997) and
Murray (1996) developed single aliquot analysis procedures that could be used
with quartz, a more ubiquitous mineral component of detrital sediments. Altho-
ugh the methods involved are different, the advantages of such measurements are
the same.
The most robust of the methods developed for quartz is known as the single
aliquot regenerative (SAR) dose procedure and has been described recently in
Murray & Wintle (2000). A brief summary is given here since many of the
results described in this paper have been obtained with this method. In essence
the procedure is very simple. In order to measure the palaeodose from a sample,
the natural OSL signal from an aliquot is measured. This measurement empties
Fig. 2. The Single Aliquot Regenerative dose method (SAR). A series of OSL measurements are made of the natural signal (LN), and the
response to various regeneration doses (L1, L2 etc.). After each measurement, the sensitivity of the sample is measured by giving a standard
radiation dose (the ‘test dose’) and measuring the OSL signal produced (TN, T1, T2 etc.). The response of the sample to dose can be plotted on
a graph of the ratio of Lx/Tx as a function of laboratory dose. The palaeodose of the sample is calculated by interpolating the value of LN/TN
onto this response curve. For this sample, the palaeodose is 22 Gy.
Luminescence dating of sediments using individual mineral grains
Fig. 3. The distribution of apparent dose in young fluvial sediments from south eastern
Australia. Measurements were made on single aliquots of quartz (from Murray et al.
1995). The samples were (a) a sub-aqueous fan in a small farm dam, (b) an inchannel
flood deposit, (c) a bed channel deposit and (d) an overbank deposit.
Fig. 4. In a mixture of bleached and unbleached grains, the probability of selecting only
well-bleached grains decreases as the number of grains on an aliquot (n) increases, and as
the fraction of the grains which are unbleached increases (from Olley et al. 1999). For
many samples the proportion of grains that contribute significantly to the total luminescence
signal is small, and hence the effective value of n may be much smaller than the actual
value (see Fig. 8).
the majority of the OSL from the sample. A laboratory dose can then be admini-
stered to the sample and the OSL signal (regenerated by that dose) measured.
This can be repeated a number of times with different regeneration doses in order
to characterise the way that the OSL signal grows with radiation dose. From this
response, the laboratory dose required to match the OSL signal obtained from the
natural measurement can be calculated (Fig. 2). This is called the equivalent dose
(DE) or the palaeodose (P).
In practice, there are additional complications. The first is that it is necessary
to apply heat to the sample prior to each OSL measurement so that all the
measurements are comparable. This preheat, along with the OSL measurement
itself, alters the response of the sample, known as its sensitivity. These changes
Fig. 5. The palaeodose obtained from measurements of many small aliquots,
with between 60 and 100 grains on each, from a range of samples from
south-eastern Australia (from Olley et al. 1998). For fluvial samples where the
degree of bleaching at deposition is lower, the range of palaeodoses is wider
than that from the aeolian sample.
Luminescence dating of sediments using individual mineral grains
in sensitivity prevented previous workers from using such an elegant and simple
solution (e.g. Duller 1991; Stokes 1994). Murray & Wintle (2000) overcame
sensitivity changes by developing a method which explicitly monitors the sensi-
tivity during a set of measurements (Fig. 2). After measurement of the OSL
signal relating to the natural dose or one of the laboratory regeneration doses (Lx),
an extra set of measurements are inserted. The aliquot is given a small radiation
dose, called the test dose, heated to 160˚C to remove any signals that would
interfere with the main OSL measurement, and its OSL signal measured (Tx). The
same test dose is used throughout a set of measurements on an aliquot. If no
sensitivity changes occurred then all values of Tx would be identical. In practice
this is not seen, and instead of plotting the raw OSL signal (Lx) to construct
a growth curve, the results are normalised by the response to the test dose (Lx/Tx)
to correct for sensitivity change.
Work by Murray et al. (1995) using an early version of the SAR procedure,
and small aliquots of only a few hundred grains, showed how modern samples
collected from a variety of fluvial depositional environments contained differing
proportions of unbleached, or incompletely bleached, grains (Fig. 3), with an
overbank deposit being the most well bleached. In this case the distribution of
palaeodoses observed from these grains are presented as histograms since it is
known that different quartz grains have very different sensitivities to dose.
Where a mixture of grains is present, the probability of obtaining a sub-sam-
ple containing only well bleached grains decreases rapidly as the number of
grains in the sub-sample increases (Fig. 4, from Olley et al. 1999). Thus in his
early work Li (1994) reduced the number of grains present in his aliquots in order
to accentuate the differences in palaeodose and this approach has also been used
successfully by Olley et al. (1998). Figure 5 shows the distribution of palaeodose
values in aliquots of quartz which contain between 60 and 100 grains (as opposed
to approximately 5000–10000 grains, which is more typical of traditional measu-
rements). These results show the difference in the luminescence signal from
young sediments in fluvial and aeolian depositional environments. A significant
number of the aliquots from the fluvial sediment contain grains whose lumine-
scence signal was not reset at deposition and so give palaeodoses that are much
larger than would be expected for a sample that was less than 5 years old.
Olley et al. (1998) suggested that a distribution of palaeodose values would
be observed where a proportion of the grains within a deposit were not fully reset
at deposition. In this situation, they showed that the best estimate for the palaeo-
dose that has accrued since the last bleaching event was obtained by taking the
average value from the 5% of the aliquots with the lowest palaeodose. This was
tested by applying the method to samples extracted from a sediment core from
the Namoi river. The results were encouraging, though there was no definite age
Lepper et al. (in press) also obtained palaeodose distributions for samples
from different depositional environments using quartz single aliquot measure-
ments. They used a deconvolution method to remove any measurement uncerta-
inties and then calculated the palaeodose relating to the latest depositional event
by taking the mean of the value between the lowest palaeodose measurement and
the peak of the distribution. Although this data gave stratigraphically consistent
results, there was no good age control with which to compare the results.
Results from individual grains
The methods used by Olley et al. (1998) and Lepper et al. (in press)
both attempt to separate from a mixed population of mineral grains the palaeo-
dose of those grains which have been bleached during the most recent depositio-
nal event, and so remove the effect of those grains which were not bleached.
A more direct method of achieving this same objective is to make measurements
of the palaeodose from individual grains.
Lamothe et al. (1994) presented palaeodoses for 15 grains of potassium
feldspar extracted from a shallow marine late-glacial sediment in Quebec. In the
case of potassium rich feldspars a significant proportion of the total dose to the
sample originates from the decay of 40K within the grain. Particularly large grains
(750–1000 µm diameter) were used in this study in order to facilitate manipula-
tion of the grains by hand during luminescence measurement. As a consequence,
at least 50% of the total dose rate arose from within the grains. The ages calcu-
lated for the 15 grains varied from 700%, to approximately 70% of the estimated
age. The presence of grains with significant age overestimates demonstrated the
benefit of single grain analysis for inadequately bleached samples. It was more
difficult to explain the presence of grains which underestimated the expected age,
but this may have been caused by a phenomenon termed ’anomalous fading’
which can affect measurements of feldspars (e.g. Spooner 1994).
Murray & Roberts (1997) were the first to obtain palaeodoses from individual
sand-sized grains of quartz. In their samples from Australia they noted that
within a single sample they observed a broad range of OSL signal intensities,
varying by several orders of magnitude. The range of palaeodose values from
a single sample was also broad, but for these sub-aerial samples the values were
consistent with a single age for the sediment.
More recently, the benefit of single grain analysis has been demonstrated very
clearly by the work of Roberts et al. (1999) at a site called Jinmium, in north-
west Australia. Jinmium is an important Aboriginal rock shelter site, with ar-
chaeological evidence of human activity. The site was first brought to prominen-
ce by Fullager et al. (1996) where thermoluminescence dates (using large aliqu-
ots of ~10 000 grains) measured on sands collected from an excavation at the
base of the rock shelter suggested that humans had arrived at this site prior to
116±12 ka, almost 50 kyr earlier than previous estimates for the first human
arrival in the continent (Roberts et al. 1994). However, the site at Jinmium is
Luminescence dating of sediments using individual mineral grains
a complex one for luminescence dating. The ‘rock shelter’ consists of a slightly
overhanging sandstone rock face. Blocks of sandstone that had fallen from the
overhanging face were encountered during the excavations at the base, mixed
together with sand that was thought to have blown into the site from the surro-
unding area. If the blocks disintegrated in situ then the grains released would not
be exposed to daylight. The presence of such grains mixed with those that were
delivered by aeolian processes would lead to an overestimate of the age. Single
grain analysis suggested that this had occurred (Roberts et al. 1998) and that if
only those grains from the younger population were included in the analysis then
a depositional age of younger than 10 ka was more appropriate.
In the last year, single grain analysis has also been used for dating fluvial
sediments. Exposure of mineral grains to sunlight during transport may occur on
a discrete basis, but in certain fluvial settings some grains may remain unble-
ached. As with Jinmium, the application of standard multiple grain analyses will
result in an age overestimate. Olley et al. (1999) have shown that such heteroge-
neous bleaching does occur in some fluvial deposits, and that single grain analy-
sis provides a means to obtain accurate depositional ages.
Practical difficulties
Analysis of single mineral grains 100–300 µm in diameter is complex.
The luminescence signal from single grains tends to be weak, and manipulating
such grains in laboratory conditions is difficult (all samples have to be handled
under dim red light prior to analysis in order to prevent loss of the light-sensitive
luminescence signal). Three methods have previously been developed to cope
with the analysis of single grains. The first, and most widely used, is to hand-pick
individual grains and mount them on standard aluminium sample holders. These
holders are 9.7 mm in diameter and are designed to accommodate many thousand
grains. However Lamothe et al. (1994) and Murray & Roberts (1997) have
successfully used this procedure for single grains. The advantage is that conven-
tional luminescence equipment can be used for the measurements. However, the
drawbacks are that it is laborious and that it makes very heavy use of instrument
A second approach has been to use an imaging system, such as an imaging
photon detector (e. g. McFee 1998a) or a charge coupled device camera (e.g.
Duller et al. 1997). Using such systems it is possible to mount many grains onto
a single aluminium sample holder and then produce a two dimensional ’image’
of the luminescence signal. Thus one can resolve the luminescence signal coming
from individual grains. This has the advantage over hand picking grains that
many grains can be analysed simultaneously. However these measurement sy-
stems are technically complex and expensive. Additionally, a critical issue is
how reliably repeated measurements can be made on a single grain. This is
Fig. 6. Diagram of the optical stimulation section of the single grain
luminescence system constructed at Risø. The path of the laser beam is shown
by the dotted lines. It is focussed using a series of three lenses, and its precise
position on the sample is controlled by moving the two mirrors. Single
mineral grains are held in a nine-by-nine array of holes drilled into the surface
of an aluminium disc. Further details of how the system works are given in
Duller et al. (1999a, b).
Luminescence dating of sediments using individual mineral grains
essential for measurement of the palaeodose from a grain, yet McFee (1998b)
calculated that the system based around an IPD had a 25% measurement uncer-
tainty. However, in spite of these problems, such systems have been employed
to measure palaeodoses (McFee 1998a).
A third approach, described by Bailiff et al. (1996), involved moving a stan-
dard aluminium sample holder under a focused laser beam so that each point on
the sample was illuminated by the laser spot in turn, and the resulting OSL signal
measured. While promising initial results were presented, the measurement time
was prohibitive; the total scanning time for a single sample holder was at least
one hour, and many such scans are required to derive palaeodoses from a single
New technology
In the last two years a new piece of equipment specifically designed
to make single grain luminescence measurements has been designed and built
(Duller et al. 1999a, b). The system has two key features. The first is that the
Fig. 7. An optically stimulated luminescence decay curve from a single grain
of quartz extracted from a Tasmanian dune sand (TNE9503). A series of
additional OSL decay curves were measured from the same grain after various
treatments, following the procedure outlined in Fig. 2. The data were used to
calculate a single aliquot regeneration (SAR) growth curve. This is shown as
an inset to the main diagram. The corrected natural OSL signal (Lx/Tx) is
shown as a solid square, while the regenerated signals used to define the
growth curve are shown as solid circles.
standard 9.7 mm aluminium sample holder has been modified so that it contains
an array of 81 holes drilled into its surface (Fig. 6). Each hole is 300 µm wide
and 300 µm deep. Each hole centre is accurately drilled so that the grains lie 600
µm apart. The second key feature is a shuttered laser beam (532 nm, 10 mW)
that is focused to a 20 µm diameter spot at the sample holder. This beam enters
the measurement chamber via two mirrors which can be moved under computer
control, and so allow the beam to be steered to any position on the sample holder.
Thus the system can direct the beam at any one of the eighty-one grains mounted
on the sample holder and stimulate OSL from that grain. This signal is then
detected using a standard photomultiplier tube. The new single grain system has
been designed to attach to an existing automated Risø TL/OSL reader so that it
can benefit from having an automated sample changer, a heater stage that allows
thermal pretreatment of the sample, and a beta source for irradiation. The total
system can perform all the measurements necessary for palaeodose calculations
under computer control without the need for an operator. Since the automatic
sample changer can cope with up to 48 samples, and each sample holder can
accommodate 81 grains, the system has a maximum capacity of just under 4000
grains, with a typical OSL measurement time of only about 200 s per disc.
Figure 7 shows an OSL decay curve measured using this automated system.
This is the OSL signal from a single 180–211 µm diameter grain of quartz. The
inset to the figure shows how the data from several such OSL measurements on
the same grain can be used to construct a growth curve and hence to calculate
the palaeodose.
Examples of single grain analysis
Grain brightness
Utilising the new equipment described above, it is possible to make
luminescence measurements of many tens or hundreds of single grains. Duller et
al. (in press) have used this instrument to measure the OSL signal from many
grains within a sample. The general feature of all of these measurements is that
there is a very large variability in the intensity of the luminescence signal obtai-
ned from different grains of the same sample. Previous authors have presented
similar data as histograms of grain brightness (e.g. McFee & Tite 1998), but this
is now considered inadequate because of the large dynamic range observed. An
alternative way to present the data is as a cumulative sum, ranking the grains in
order of descending brightness, and then plotting the cumulative light sum as
a function of the proportion of the brightest grains involved (Fig. 8). If all grains
gave the same OSL signal then a diagonal line would be plotted from the origin
to the upper right hand corner of the diagram. In practice all samples will fall
Luminescence dating of sediments using individual mineral grains
above and to the left of the line. The further that a sample plots away from the
‘ideal’ diagonal line, the less even the distribution of signal within the different
grains making up the sample. For instance, in samples BA14 and RBM2 (coastal
aeolian sands from southern Africa), over 95% of the OSL signal originates from
less than 5% of the grains, and the majority of the grains play a minor role in the
overall signal. In contrast, WIDG8 (an aeolian sand from northern Australia)
contains many grains that contribute.
Figure 4 showed that where a mixture of well-bleached and unbleached gra-
ins existed, the probability of selecting only well-bleached grains decreased
rapidly as the number of grains increased. For multiple grain work this will give
scatter in the palaeodose values obtained. If one compared the multiple grain
Fig. 8. The proportion of the total OSL light sum from a set of grains plotted
as a function of the proportion of the brightest grains that are used. For
a population of grains which all have the same brightness, the line would run
diagonally from bottom left to top right. For all natural samples the line plots
to the left of this. Data from a range of samples are shown. The number of
grains measured from each sample is shown in brackets. Details of the
samples are given in Duller et al. (in press).
behaviour of two samples, such as BA14 with a few very bright grains and many
dimmer ones, and WIDG8 with many similarly bright grains, they would be very
different since the effective number of grains on an aliquot of equal mass is very
different. This is an additional reason why single grain analysis is preferable to
multiple grain analysis where there is a mixture of well-bleached and unbleached
Palaeodose distributions
Figure 9 shows a set of palaeodoses obtained from 408 grains of
a dune sand from north-east Tasmania. The sample dates from the last glacial
maximum, and has not undergone any post-depositional reworking. An important
consideration when analysing single grain data is how to present the results. As
shown above, grains of quartz from a single sample may have OSL signals which
vary by over two orders of magnitude. As well as affecting the multiple grain
behaviour, this will influence the precision with which the palaeodose can be
calculated. For the brighter grains the uncertainty may be 5%, while for dimmer
grains it might be 100%. For such data it is unreasonable to present it as a histo-
gram since this implicitly assumes that each data point should be weighted
equally. Two alternative methods of presentation are possible. The first is to
produce a probability density function (PDF). This is a weighted histogram whe-
re each palaeodose is represented by a gaussian curve whose peak is at the
palaeodose value, but whose height is inversely related to the precision with
which the palaeodose is known. Hence the result from a grain whose palaeodose
is well known is represented by a narrow, high peak, while a grain whose palaeo-
dose is poorly known is represented by a low broad peak. The results for all
grains are then summed to produce a single probability density function (Fig. 9a).
This approach has been criticised because it makes it impossible to discern the
influence of an individual data point upon the overall distribution. Instead, Gal-
braith (1990) suggested using a radial plot (Fig. 9b). On this graph, each data
point is represented discretely. Full details of how such plots are constructed are
given by Galbraith (1990). The essential points are that the greater the precision
with which a palaeodose is known, the further it is plotted to the right of the
graph. The difference between some average palaeodose value and the value for
the specific grain, divided by the standard error on that specific palaeodose
dictates the vertical position of the point on the graph. A consequence of the
quantities that are plotted is that any points lying on a line drawn through the zero
point on the y-axis all have the same palaeodose. Thus a radial scale can be
added on the right hand side of the graph, marked with values of palaeodose.
An additional advantage of the radial plot is that one can draw two parallel
lines from the values +2σ and –2σ on the y-axis to intersect the radial axis on
the right hand side. Any data point whose palaeodose is consistent, within two
Luminescence dating of sediments using individual mineral grains
Fig. 9. Palaeodose measurements from 408 grains of quartz extracted from
a Tasmanian dune sand (TNE9503). The palaeodose values are plotted (a) as
a probability density function and (b) as a radial plot. The way in which such
plots are constructed is described in the main text.
Fig. 10. Palaeodose measurements from 37 grains of quartz extracted from
a frost-wedge cast, Denmark (992101). The palaeodose values are plotted as
(a) a probability density function and (b) a radial plot.
Luminescence dating of sediments using individual mineral grains
standard deviations, with the mean palaeodose value used to construct the radial
plot will fall within the band defined by these two lines. In the case of the aeolian
sand from Tasmania, 81% of the grains do indeed fall within this band, demon-
strating that the grains from the sample do yield a set of palaeodose values which
are consistent with a single value of 22.8±0.4 Gy, implying that the sample was
well bleached at deposition.
In contrast, analysis of single grains from a sand infilling a frost-wedge cast
(sample 992101) give a wide range of palaeodose values, and the radial plot
(Fig. 10b) shows that only 51% of these are consistent with the mean value
within two standard deviations. This sample clearly consists of at least two
populations with a peak in the probability density function at approximately 50
Gy (Fig. 10a).
Luminescence dating methods have been applied successfully to sedi-
ments from a wide range of Quaternary sites over the last two decades. The
development of single aliquot procedures for palaeodose determination has in-
creased the throughput of samples, and reduced the analytical uncertainties.
The most successful depositional environments for luminescence dating are,
not suprisingly, those in which the probability of grains being exposed to daylight
at deposition is large, such as coastal and desert dunes (Wintle 1993) and loess.
In such environments it is reasonable to assume that all grains have been equally
bleached. However, there are a wide range of environments in which it is possib-
le that grains have been adequately exposed to daylight, but it would be unwise
to assume this to be the case. Sediments deposited by fluvial, glacio-fluvial and
mass movement processes are likely to consist of a complex mixture of grains,
with only a proportion having been completely exposed. In such situations the
luminescence age calculated using standard multiple grain procedures will be an
overestimate. A variety of methods have been developed which will identify
such complex situations from single aliquot data, but which are unable to provide
an indication of the extent of the overestimate. In such situations aliquots with
a restricted number of grains may provide an estimate of the true palaeodose.
A more direct method is to analyse single grains in order to directly differentiate
between those grains which have been bleached and those which have not.
The development of single aliquot and single grain analytical procedures
provide a new avenue of research in luminescence dating. The ability to analyse
the luminescence properties of individual mineral grains within a sample widens
the range of depositional environments in which the method can be applied, and
provides far greater information about the sample than was available previously.
Routine analysis of single grains is also being made practical by the development
of instrumentation specifically designed for this purpose.
Acknowledgements: This work is funded by the Danish Natural Science Research Council
(9701837) and NERC (GR3/E0087). Dr Helen Roberts suggested valuable improvements to an
early version of the manuscript. The support and encouragement of Lars Bøtter-Jensen and Ann
Wintle throughout our pursuit of single grain measurements has been greatly valued.
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... There was obvious inversion of the OSL age of 20 ± 40 a BP at 82 cm depth, possibly due to disturbance and pollution; therefore, the present study disregarded this record. The plant root system may be an occasional disturbance to the OSL age of the 7D layer and can't be ruled out [39]. But for the AMS-14C age of the 4Pd layer, there are large carbonaceous fragments reserved in situ, so the disturbance is less likely. ...
... Grain-size analysis was conducted at the Key Laboratory of Environmental Evolution and Resource There was obvious inversion of the OSL age of 20 ± 40 a BP at 82 cm depth, possibly due to disturbance and pollution; therefore, the present study disregarded this record. The plant root system may be an occasional disturbance to the OSL age of the 7D layer and can't be ruled out [39]. But for the AMS-14C age of the 4Pd layer, there are large carbonaceous fragments reserved in situ, so the disturbance is less likely. ...
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The Wutou section, hereinafter referred to as “WTS”, lies in Jiangping, Guangxi Province, China (21°32′8.25″ N, 108°06′59.9″ E; thickness of 246 cm) and consists of fluvial-lacustrine facies and dune sands of the Late Holocene. This study reconstructed the evolution of storm surges along the coast of the Beibu Gulf, Guangxi over the Little Ice Age, based on three accelerator mass spectrometry (AMS)-14C, optically stimulated luminescence (OSL) dating ages, and the analyses of grain size and heavy minerals. The analysis results indicated that the storm sediments interspersed among aeolian sands, lagoon facies, and weak soil display a coarse mean grain size and poor sorting. The storm sediments also show high maturity of heavy minerals and low stability resulting from rapid accumulation due to storm surges originating from the land-facing side of the coastal dunes. Records of seven peak storm surge periods were recorded in the WTS over the past millennium and mainly occurred after 1400 AD, i.e., during the Little Ice Age. The peaks in storm surges, including the 14Paleostrom deposit (hereinafter referred to as “Pd”) (1425–1470AD), 10Pd (1655–1690AD), 6Pd (1790–1820AD), and 4Pd (1850–1885AD) approximately corresponded with the periods of minimum sunspot activity, suggesting that the periods of storm surge peaks revealed by the WTS were probably regulated to a great extent by solar activity.
... This SAAD technique for quartz (and feldspar) requires correction for sensitivity change during read outs as has been described elsewhere and has been further improved and produced fundamental results (Duller 1994 a, b;Liritzis et al., 1994;1997b;2001Duller & Murray, 2000). ...
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The optical stimulated luminescence (OSL) dating is a steadily developed method in the fields of archaeology, geoarchaeology and earth sciences. The trapped electrons in lattice defects of suitable minerals during irradiation by natural radioisotopes throughout the time and the emitted luminescent light after agitation by optical radiation in the lab (or by daylight), are physical processes which determine the total equivalent (to a laboratory dose) dose (in Grays). The intentional or accidental light exposure to light and the thermal agitation sets luminescence clock to zero for a mineral within a material to be dated. Excellent publications (either articles or books) and reviews on the OSL dating have been made. The objective coverage of essential novel applications and basic research accompanied by an ethic unbiased pace is a prerequisite for scientific integrity of every publication. A recent Nature Reviews Method Primers on OSL dating of quartz was the stimulus for the present constructive and supplementary information and serves also as a plea for fairness.
... The technique can be hampered in situations in which (1) the proper species of quartz are not present in the deposits, and (2) for floods where the transported sediment was not bleached by exposure to light, either because of high turbidity levels during the flood or because the flood occurred at night (Medialdea et al., 2014;Smedley and Skirrow, 2020). Developments in OSL instrumentation are reducing the sample size to individual quartz and feldspar grains (Bøtter-Jensen et al., 2000;Duller and Murray, 2000). Under appropriate conditions, OSL dating is an important tool, especially for deposits (1) containing little or no organic materials, (2) older than the range of radiocarbon dating (>40,000 years), or (3) younger than 300 years when radiocarbon dating is imprecise. ...
This article reviews paleohydrologic techniques for estimating the magnitude and frequency of past floods using geological evidence. Quantitative paleoflood hydrology typically involves several activities: (1) identification and analysis of physical evidence of past floods, such as slackwater deposits, boulder bars, and tree scars; (2) geochronological studies to determine the ages of features left by past floods, commonly by radiocarbon and luminescence dating; (3) estimating peak discharges associated with flood evidence from hydraulic calculations; and (4) incorporating information on the number, magnitude and timing of large past floods into flood frequency estimates, thereby improving quantile estimates for large and rare floods.
... L'innovation principale de l'approche récemment mise en pratique à Bordeaux pour les mortiers consiste à l'emploi systématique de la technique dite « single grain » (SG-OSL) 39 . Cette méthode d'analyse permet de détecter la luminescence de chaque grain individuellement et donc de distinguer les différents degrés de blanchiment des grains dont seulement une partie porte une information chronologique sur la construction du bâtiment. ...
... With the aim of suggesting a universal dating procedure that would be applicable on all mortars, involving those affected by insufficient light exposure, we introduced a methodological innovation that consists in the systematic use of the so-called "single grain" OSL technique for dating mortars (SG-OSL; for the technique: (Duller & Murray, 2000); for the application on mortar: ). This approach, which detects the luminescence of each grain individually, is the outcome of recent technological advancements and has become relatively frequent in geological dating applications, but is still rarely employed in mortar dating. ...
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This paper deals with new strategies for dating the construction of ancient monuments, one of the most topical issues in archaeology. Our approach is demonstrated by the study of an emblematic early medieval Basilica Saint Seurin in Bordeaux whose oldest building phases have never been well-understood and dated before due to the lack of written sources and archaeological findings. We mainly focus on the analyses of mortar as an omnipresent and non-recyclable material whose making is undoubtedly contemporary to the building process. For the first time, we integrated a novel, recently validated protocol for dating historical mortar through optically stimulated luminescence using the « single grain technique » (SG-OSL) into archaeological research. The present work arises from close and continuous collaboration between archaeologists and archaeometers both in situ and during post-excavation analyses. SG-OSL dating of mortar, as the most innovative aspect of the study, was combined with mortar characterization, radiocarbon dating of charcoals and partly also with ar-chaeomagnetic and thermoluminescence dating of bricks for a cross-check of chronological data. We identified and dated several independent building phases in the crypt of the present church where mortar was the only building material preserved. By combining physical dating methods with stratigraphic constraints based on archaeological interpretations, all the findings were used to construct a chronological model that proves continuity in occupation of the site between the 5th and the 12th centuries, reflecting its high cultural and symbolic value. By the inter-connection of mortar dating by SG-OSL with archaeology and other fields of archaeometry, we set up a renewed interdisciplinary working model for building archaeology that opens interesting perspectives for the future of this research field.
The Late Roman Antiquity walls of Le Mans in northwest France are one of the most representative and preserved examples of the urban fortifications that developed in the Roman provinces of Gaul and Germany at this period. Because of a lack of reliable chronological data, the construction of the walls was poorly dated, which made unclear its historical context. The main objective of this study was to reassess the date of construction using several methods on a well-preserved sector (sector 11) of the Late Roman Antiquity walls. The dated masonries, thoroughly studied in building archaeology beforehand, are characterized by an alternating of stone and brick courses. Sampling focused on one hand on mortars with four radiocarbon dates (14C) and six single-grain optically stimulated luminescence (SG-OSL) dates, and on the other hand on bricks with two OSL dates (quartz fine grain technique) and one archaeomagnetic date on a set of 104 bricks. The consistency between the dates on the two types of materials discards a possible reuse of the bricks from former Roman buildings. They were produced for the construction of the walls with the presence of several types of bricks likely reflecting a supply from at least two workshops. The dating program in the sector 11 also included 21 14C dates and six SG- OSL on protohistoric structures, Early Empire masonries and large medieval buildings, in order to investigate the evolution of the area over the long-term and to better constrain the chronology of the Late Roman Antiquity walls in Bayesian modelling. The chronological model (Chronomodel software) dates this construction between 301 and 423 CE at 95% of confidence. This date clearly excludes that the Late Roman Antiquity walls were built during the crises of the 3rd century, as previously thought, but rather in the stable political and economic context of the 4th century.
Technical Report
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Paleoflood studies are an effective means of providing specific information on the recurrence and magnitude of rare and large floods. Such information can be combined with systematic flood measurements to better assess the frequency of large floods. Paleoflood data also provide valuable information about the linkages among climate, land use, flood-hazard assessments, and channel morphology. This document summarizes methods and techniques for the preparation, gathering, evaluation, and interpretation of paleoflood information, including uncertainties, especially with respect to new statistical approaches available to efficiently use such data. We summarize best practices and strategies for assessing and mitigating uncertainties and provide guidelines on appropriate technical review of paleoflood analyses based on project goals and requirements.
Quaternary alluvial and colluvial sediments infill major river valleys and form alluvial fans and colluvium-filled bedrock depressions on the range fronts and within the Mount Lofty Ranges of southern Australia. A complex association of alluvial successions occurs in the Sellicks Creek drainage basin, as revealed from lithostratigraphy, physical landscape setting and optically stimulated luminescence (OSL) ages. Correlation of OSL ages with the Marine Oxygen Isotope record reveals that the alluvial successions represent multiple episodes of alluvial sedimentation since the penultimate glaciation (Marine Isotope Stage 6; MIS 6). The successions include a penultimate glacial maximum alluvium (Taringa Formation; 160 ± 15 ka; MIS 6), an unnamed alluvial succession (42 ± 3.2 ka; MIS 3), a late last glacial colluvial succession within bedrock depressions (ca 15 ka; MIS 2) and a late last glacial alluvium (ca 15 ka; MIS 2) in the lowest, distal portion of Sellicks Creek. In addition, the Waldeila Formation, a Holocene alluvium (3.5 ± 0.3 ka; MIS 1), and sediments deposited during a phase of Post-European Settlement Aggradation (PESA) are also identified. The age and spatial distribution of the red/brown successions, mapped as the Upper Pleistocene Pooraka Formation, directly relate to different topographic and tectonic settings. Neotectonic uplift locally enhanced erosion and sedimentation, while differences in drainage basin sizes along the margin of the ranges have influenced the timing and delivery of sediment in downstream locations. Close to the Willunga Fault Scarp at Sellicks Creek, sediments resembling the Pooraka Formation have yielded a pooled mean OSL age of 83.9 ± 7 ka (MIS 5a) corroborating the previously identified extended time range for deposition of the formation. Elsewhere, within major river valleys, the Pooraka Formation was deposited during the last interglacial maximum (128–118 ka; MIS 5e). In general, alluviation occurred during interglacial and interstadial pluvial events, while erosion predominated during drier glacial episodes. In both cases, contemporaneous erosion and sedimentation continued to affect the landscape. For example, in the Sellicks Creek drainage basin, which lies across an actively uplifting fault zone, late glacial age sediments (MIS 2) occur within the ranges and near the distal margin of the alluvial fan complex. OSL dating of the alluvial successions reported in this paper highlights linkages between the terrestrial and marine environments in association with sea-level (base-level) and climatic perturbations. While the alluvial successions relate largely to climatically driven changes, especially in major river valleys, tectonics, eustasy, geomorphic setting and topography have influenced erosion and sedimentation, especially on steep-sloped alluvial fan environments. • KEY POINTS • Luminescence dating of the Sellicks Creek alluvial fan complex reveals that sedimentation occurred predominantly during the later stages of glacial cycles accompanying lower sea-levels than present. • Luminescence dating confirms that the stratigraphically lower portions of the Pooraka Formation are beyond the range of radiocarbon dating. • Upper Pleistocene alluvial fan sedimentation at Sellicks Creek correlates with pluvial events in southeastern Australia.
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Determining the most appropriate luminescence protocol, coupled with suitable data processing methods, for dating recently deposited sediments (<200 years) is important for identifying episodes of sediment movement and interpreting historical landscape dynamics. Issues of partial bleaching, dim luminescence signals and the incorrect application of rejection criteria, can lead to inaccurate and imprecise ages of recent sediment deposition. This study first compares the performance of quartz optically stimulated luminescence (OSL) and K-feldspar post-IR IRSL (pIRIR) measurements in a series of dose recovery preheat plateau, bleachability and remnant dose tests. Sediments of known historical age are used to identify the most suitable aliquot size and age model choice for further application on near-surface aeolian dune sediments from the Nebraska Sandhills. Results show that the ideal conditions for measuring these aeolian sediments are small aliquots (2 mm) of either quartz or K-feldspar coupled with the relevant protocols (OSL130 pIRIR170) and the unlogged-CAM and unlogged-MAM respectively. Results of 4 ± 7 years (quartz) and 4 ± 8 years (K-feldspar) are in excellent agreement with aeolian sediments of known age 5–6 years. Additionally, we find a revised set of rejection criteria is useful for accurately identifying the appropriate aliquots or grains for reliable age estimation. Sensitivity testing of recuperation rejection criteria highlights the caution that should be taken to avoid arbitrarily applying rejection criteria and biasing towards age overestimations.
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Reviews new ideas concerning the luminescence signals that can be derived from quartz and feldspars. The advantages of stimulating luminescence by the application of optical radiation, rather than heat, are discussed. The application of TL and IRSL techniques to colluvial deposits in southern Africa is discussed with reference to a preliminary study of four samples from sediments in northern Natal. Laboratory procedures are given in some detail and indicate the complexity of age determinations by these methods. The TL age determinations are significantly higher than those obtained by IRSL. -from Authors
In the optical dating of sediments it is usually assumed that the optically stimulated luminescence (OSL) signal has been completely reset by light exposure prior to burial; this assumption is often not valid. One approach to testing, and perhaps circumventing, this assumption is to examine the apparent date of last exposure to daylight of individual sediment grains. This paper reports on the application, for the first time, of 2 new measurement protocols to the estimation of the radiation dose received during burial for individual quartz grains from an aeolian deposit of known age (10,000 year old), which is considered likely to have been completely reset by sunlight at deposition. Additive-dose (laboratory doses added to the burial dose before OSL measurement) and regenerative-dose (doses added after measurement of OSL from burial dose) single-aliquot protocols are applied to 28 and 25 individual grains, respectively; each grain provides an independent estimate (De) of the burial dose. The average De from the additive-dose protocol (21.8±1.1 Gy) is in good agreement with the average from the regenerative-dose protocol (23.8±1.0 Gy). Both agree well with: (1) 13 multiple-grain regenerative-dose single-aliquot measurements, each on 1 mg sub-samples, of 23.9±0.3 Gy; (2) 9 multiple-grain additive-dose single-aliquot measurements, also on 1 mg sub-samples, of 22.4±0.7 Gy; and (3) one previously published multiple-aliquot additive-dose estimate of 23.5±0.6 Gy using 52 sub-samples, each of 5 mg. The resulting optical ages are in good accord with 14C and thermoluminescence age determinations. The distribution of equivalent doses in the single grains is, however, unexpectedly large (σ≈23% of mean De), given the very likely complete resetting of the OSL signal at deposition. Possible reasons are discussed, and it is concluded that heterogeneity in beta dosimetry is the most likely explanation. The single-grain optical dating protocols reported here allow a detailed examination of the dose distribution in very small samples. Thus, they should enable accurate dates to be obtained for sediments and soils that contain poorly bleached or mixed-age components, as well as deposits in which quartz grains are present in extremely low abundance.
Optically stimulated luminescence (OSL) is used widely for reconstructing past radiation exposure, either in connection with accidental release of radionuclides into the environment, or for dating the time since geological materials were deposited. Measurements of the optically stimulated luminescence properties of crystals are conventionally undertaken on groups of many hundred to many thousand sand-sized (90-300mum) grains. However, it has long been known that different grains may have different luminescence properties (e.g., sensitivity to dose) and that more information could be gained if single grains could be measured separately, and thus avoid the effect of averaging. Here we describe an automated system that makes the routine measurement of OSL of a large number of single grains feasible for the first time. The concepts underlying the design are described, and initial measurements demonstrate that a reproducibility of 3% can be achieved in repeated OSL measurements of a single grain of Al2O3:C. Measurements on a geological quartz sample demonstrate that the system can also analyse natural samples.
An imaging photon detector (IPD) has been used to image the infrared stimulated luminescence (IRSL) from individual feldspar grains using an array of 24 infrared diodes. The use of an IPD allows individual grains to be resolved from within an image of a large mass of grains and this minimises the amount of light exposure grains must receive, as picking of individual grains is unnecessary. The IRSL was sufficiently bright to allow the single aliquot additive equivalent dose (ED) method to be used on single feldspar grains, and individual EDs were found for grains from several feldspathic sediments. The weighted EDs from these grains agreed with the ED which had been obtained through multiple aliquot additive measurements of the bulk samples. Single grain IRSL EDs measured with the IPD provide a good method for detection of, and correction for, insufficiently bleached grains.
Two types of poorly bleached sediment are differentiated and the applicability of various luminescence dating methods is assessed. The methods are applied to three fluvioglacial sediments from Scotland. It is shown that it is possible to detect sediments that contain a mixture of well bleached and unbleached grains using the single aliquot additive dose method or the analysis of the scatter in aultiple aliquot additive dose growth curve.
Relatively few sites have been identified in Scotland that contain records that predate the peak of the Late Devensian glaciation (ca. 18 ka BP). Where such records have been identified they have assumed a key role in developing our understanding of climate change in Scotland. Radiocarbon dating has been the primary chronological tool applied to these sites, but the reliability of age determinations in excess of 30 ka is questionable. In this paper we describe the results of a programme which applies luminescence dating methods to eight key pre-Late Devensian sites in Scotland. Methods were developed and tested to enable the identification of samples for which luminescence dating was only able to provide an upper limit on the age because of the limited light exposure that the sample experienced at deposition. These new approaches were applied to four sites where independent age control was available; this meant that the choice of sediments from test sites was restricted to those sediments which were formed in the last 40,000 years and hence were amenable to radiocarbon dating. The luminescence age estimates for the control sites and the pre-Late Devensian sites are discussed within the context of any existing chronological control on a site by site basis.
An Imaging Photon Detector (IPD) can be used to obtain quantitative TL or IRSL images of the luminescence distribution in a mineral or pottery slice, or to measure the luminescence from individual mineral grains. Properties of the IPD itself, such as resolution and speed of response to the photon flux are important characteristics which will have important consequences for experimental design. However, the range of experiments that are possible in imaging studies are also dependent on the experimental system as a whole, such as the optics used, and the detector characteristics. In this paper the behaviour of such a system, using an IPD as the detector, is described, The consequences for the experimental methodologies adopted are also discussed, as are the range of measurements that are achievable from such a system.